1. Field of the Invention
The present invention relates generally to a vehicle steering system and more specifically to a method and apparatus for hydraulically assisting a steering system.
2. Description of Related Art
A typical automotive vehicle generally utilizes a steering member, such as a steering wheel, connected to the vehicle wheels through a rack and pinion mechanism. As the vehicle operator manipulates or moves the steering wheel, the corresponding vehicle wheels turn accordingly.
In a rack and pinion steering mechanism rotation of the steering wheel rotates a pinion gear coupled to the steering wheel through a steering column or shaft. The pinion gear engages a rack gear disposed between the steerable wheels. Accordingly, rotational motion of the steering wheel is translated into lateral motion that correspondingly turns the vehicle wheels. In order to reduce the amount of vehicle operator effort required to rotate the steering wheel and correspondingly turn the vehicle, many steering systems include a power assist system. The power assist system assists the vehicle operator with rotation of the steering wheel to overcome road load forces occurring during vehicle operation.
Conventional power assist steering systems typically employ either hydraulic power assist or electric power assist mechanisms. Such power assist systems vary the amount of power assistance depending upon the speed of the vehicle and the amount of effort applied by the vehicle operator to turn the steering wheel. Current hydraulic power assist steering systems typically utilize a metering valve integrated into the input/pinion shaft of a steering gear. As the vehicle operator applies an input through the steering wheel, the valve directs and meters the flow of hydraulic fluid to provide proportional steering assist. Typically, such a system requires expensive high-precision manufacturing processes to produce a proper metering valve. Furthermore, such a system typically has a single, pre-defined boost or assist curve that depends on valve geometry. Such systems typically do not provide sufficient assist across the full range of vehicle speeds. For example, when steering the vehicle at slower speeds such as during parking, or when the vehicle is stationary, a greater amount of assist is usually needed. At higher vehicle speeds, the system may provide too much assistance thus reducing the feel of the steering system.
In addition, the power steering pump, used to generate the hydraulic fluid pressure that provides the assist force, typically runs off of the vehicle engine; i.e. the pump is typically driven via a belt connected to the output shaft of the engine. Accordingly, the pressure generated by the pump varies with engine speed or engine revolutions. When the engine is operating at a low speed or RPM, the power steering pump must provide a sufficient supply of pressurized hydraulic fluid to assist the vehicle operator in steering the vehicle. Accordingly, the power steering pump is tuned or configured to provide a suitable power assist at low speeds or RPMs. Correspondingly, when the engine is operating at high engine speeds or RPMs, the power steering pump operates at a high output using additional energy from the vehicle engine while needing a bypass or bleed circuit to reduce excess fluid pressure and flow. Thus, the power cost of the pump is highest at high engine speeds; i.e., engine speeds normally associated with high vehicle speeds, a condition not normally requiring an increased level of steering assistance. In addition, high output of the power steering pump may result in too much steering assist at high speed thereby reducing the feel of the steering system.
With electric power assist mechanisms, an electric motor is used to provide a steering boost or assist. The electric motor can be attached at a variety of positions on the steering assembly. Sensors detect the motion of the steering column and provided an input to the electric motor. Typically, some type of software or other computer program tunes the characteristics of the electric power steering system to provide steering feel to the vehicle operator. Often, these systems have issues with feel in that the feel may be too light or does not approximate. the force needed to turn the vehicle. In addition, limits on the vehicle electric system limit the size of the motor used to provide the power assist. Further, the software may have to be complex or be based on complex model based control algorithms to improve the electric motor efficiency. Such systems must address a multitude of key system parameters such as, motor currents, motor wire positions and motor supply voltage fluctuations along with driver input and feel.
Accordingly, it is desirable to provide a steering assist that controls and provides a variable or multiple boost curves which improves vehicle steering feel across the full range of vehicle speeds.